1. Field of the Invention
This invention relates to the field of resin blends containing partially hydrolyzed ethylene-vinyl acetate (PHEVA) resin as an impact modifier for brittle, hydrolyzed ethylene-vinyl acetate (HEVA) resin containing at least about 40 weight percent interpolymerized vinyl alcohol and no more than about 10 weight percent interpolymerized vinyl acetate.
2. Description of the Prior Art
Hydrolyzed ethylene-vinyl acetate resins containing in interpolymerized form from about 2 to about 60 weight percent ethylene, from about 98 to about 40 weight percent vinyl alcohol and no more than about 10 weight percent of vinyl acetate provide moldings which exhibit excellent oil-resistance, stiffness, hardness, tensile strength, flexural strength, compression strength, abrasion resistance and antistatic charge. Unfortunately, however, these resins possess such a low level of impact-resistance that they are for the most part excluded from engineering applications where high impact strength is an indispensable requirement. Thus, for example, brittle, hydrolyzed ethylene-vinyl acetate resins of the foregoing type, due to their poor impact-resistance are generally unsuitable molding materials for such important commercial applications as electric switch covers, knobs or cabinets, machine parts such as pulleys, rollers, bars, gears, cams or bobbins and automotive parts subject to dynamic stress loads.
It has been acknowledged by Takida et al. (U.S. Pat. No. 3,887,649) that past attempts to remedy the low impact-resistance deficiency of hydrolyzed ethylene-vinyl acetate resins by incorporating therein a synthetic rubber such as styrene-butadiene copolymer or acrylonitrile-butadiene copolymer have not met with success. For example, while Gardiner et al. (U.S. Pat. No. 3,585,177) and Coates et al. (U.S. Pat. No. 3,549,727) propose the use of nitrile rubbers as impact modifiers for brittle, hydrolyzed ethylene-vinyl acetate resin, experience with these modifiers in simple blends of the hydrolyzed resin has been unfavorable due to the tendency of the blends to separate into the component resins and/or develop an unacceptable degree of opacity. Takida et al. also acknowledge that while blends of hydrolyzed ethylene-vinyl acetate resin and ethylene-vinyl acetate resin improves impact resistance at normal temperature "to some extent", such blends remain unsatisfactory since the tensile strength and flexural strength are remarkably reduced and impact-resistance at low temperature is insufficient. Takida et al. propose the improvement of the impact-resistance of hydrolyzed ethylene-vinyl acetate resins by the addition thereto of a block-copolymerized polyester-polyether elastomer having a melting point of crystalline region of 150.degree. to 230.degree. C. and an impact resilience of not less than 50%. The related resin blends of Japan Kokai No. 75-18, 553 (1975) (Chem. Abs. 83 8044) cites the modification of hydrolyzed ethylene-vinyl acetate resin with a polyester elastomer. The hydrolyzed ethylene-vinyl acetate resin impact modifier of Japan Kokai No. 74-33,941 (1974) (Chem. Abs. 84 60595) is polyurethane while in Japan Kokai No. 76-05, 354 (1976) (Chem Abs. 84 136723) it is a mixture of low density polyethylene and glycerol. Japan Kokai No. 75-23, 443 (1975) (Chem. Abs. 83 80478) describes transparent, impact-resistant moldings of hydrolyzed ethylene-vinyl acetate resin modified with a butadiene-methyl methacrylate-styrene graft terpolymer. The latter resin, being organic solvent sensitive, can easily be lost under molded part use conditions and for this reason is an undesirable ingredient when the hydrolyzed ethylene-vinyl acetate resin blend is employed in molding operations. Nohara et al. (U.S. Pat. No. 3,882,259) describe a laminated sheet in which the ethylene-vinyl alcohol copolymer intermediate layer contains an ionomer resin and/or an ethylene-vinyl acetate copolymer for the purpose of improving the resistance to permeation of gas and moisture and to improve the bonding force between the layers. Patentees are completely silent with respect to adding an impact-resistance modifier. Fuksushima et al. (U.S. Pat. No. 3,890,267) describe glass fiber-modified hydrolyzed ethylene-vinyl acetate resins; related materials are the so-called "GL Resins" (Kawaguchi et al., JAPAN PLASTICS, November-December 1974, pages 6-14 and January-February 1975, pages 11-15). In addition to lacking transparency, the impact-modifying effects of the glass fibers are to some extent lessened when the resins are processed by such techniques as injection molding as the high shear forces occurring in the extruder result in reducing the length of the glass fibers and hence reducing their contribution to impact-resistance.
The addition of several of the known impact modifiers is disadvantageous either in respect of negatively affecting an important property of the host resin, e.g., transparency, and/or being unsuited to certain processing operations, e.g., molding. Moreover, for the manufacturer of hydrolyzed ethylene-vinyl acetate resin, the addition of an impact modifier which is altogether entirely unrelated to the host resin is usually less than an ideal solution since it often requires maintaining inventory of a material which must be obtained from an outside source and as such, is beyond the manufacturer's direct control.